These mystery machines are about one-quarter the size of NASA’s old space shuttles and they can land automatically on a runway.

While the plane itself is not especially secretive — NASA is very open about the spaceplane and the fact that it launched — it’s what it’s doing up there that has the U.S. government’s lips sealed.

The military isn’t saying much if anything about this new secret mission. But one scientific observer, Harvard University’s Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculates the spaceplane is carrying sensors designed for spying and likely is serving as a testbed.

While updates about the spaceplane won’t be coming from NASA, amateur astronomers have tracked the previous X-37B flights.

The flight and its Atlas V rocket were handled by the United Launch Alliance (ULA) which is a 50/50 joint venture between Lockheed Martin and Boeing. The rocketry company has over 3,900 employees.

The two previous secret flights were in roughly 200-plus-mile-high orbits.